AZO COUPLING COMPONENTS
The first practical azoic coupling component was b-naphthol. The azoic dyeings from b-naphthol had many limitations, such as the lack of substantivity of b-napthol for cotton, the narrow range of shades obtainable from a single coupling component and the poor fastness to light and rubbing. In 1912, naphthol AS-arylamides of BON Acid was introduced as coupling components having definite cotton affinity and producing azoic dyeings of greatly improved brilliance and fastness.The group of azoic coupling components of this is known as the Naphthol AS series. These are generally made by condensing BON Acid with an aromatic amino compound with or without one or more of chloro, nitro, methyl, methoxy, etc. groups in different positions with respect to the amino group. Most of the naphthols are made from substituted aniline and naphthylamines.
RAPID FAST COLOURS
The composition of various Rapid Fast Colours was stated to be as follows; the base of which the anti-diazotate is used, and the Naphthol, are both indicated in table 3.3
m-Nitro Aniline (Fast orange R)
m-Nitroaniline is used in organic synthesis and as a dye intermediate..
Agitate 340 ibs dinitrobenzene in 5000 L water in a wooden vat. Raise the temp. to 85°C. Add filter solution made from 280 ibs flake sodium sulphide with 140 ib sulphur in 1000 litres water during 1 hour. Agitate the reaction for one hour. Filter the above solution into wooden vat, recover unreacted sulphur. Cool the above solution, filter the crystals of m-nitro aniline. These crystals are dissolved in hydrochloric acid and then precipitated in alkali solution.
In a modification of above process, agitate 340 ibs dinitrobenzene in 5000 L water at 85°C. Add filter solution made from 400 ibs rock sulphide and 350 ibs hydrochloric acid in 1500 litres water.
Yield - 80 to 85%
Colour - Yellow crystalline compound. M.P - 114°C
Boiling pt.- 286°C
Density20 - 1.430.
Soluble in 875 part of water, 15 parts of alcohol. 16 parts of either, very soluble in acids.
O-CHLOROANILINE (FAST YELLOW G, GC)
O-Chloroaniline is manufactured by the reduction of O-nitro-chlorobenzene. The reduction vessel is provided with acid proof bricks to withstand the erosive,conditions encountered due to the handling of iron borings. Take 13 litre of hot water in reduction vessel. 10.5 kg Iron powder is charged under stirring. The type of Iron borings suitable for reduction is very important. It is desirable to test the suitability in laboratory before accepting these borings in the plant. The particle size of iron borings is important and it should be fine and uniform. 200 gm acetic acid is added, 10 kg orihonitrochlorobenzene is added to the reduction vessel at 90°C slowly preferably in about 4 hours. Add 200 gm acetic acid during this period.
After completing the addition of orthonitrochlorobenzene the reaction is further continued for 6 hours under-reflux. Every hour, the sample is withdrawn to test for free iron content and pH. The pH is maintained between 5 and 6. After completion of reaction, it is made alkaline by adding caustic.
Toluene about (17.5 lit) is added and stirring is continued for one hour. The material is filtered hot. The product remaining in the Iron sludge is again extracted in toulene. The filtrate is transferred to layer separation vessel. The toluene extract is steam-distilled and most of the toluene is recovered under atmospheric pressure.
The final product distillation is performed under vacuum % yield based on ONCB is 85-90% approx.
1. Formic Acid can be used for the reduction in place of Acetic
2. Product can be extracted by using water or steam instead
Properties of O-Chloroaniline
Appearance: Colourless to pale reddish liquid. Boiling
Point: 208-210ºC approx.
Specific Gravity: 1.21320/4
Solubility: Miscible with alcohol and ether, insoluble in water.
O-ANISIDINE (FAST RED BB)
Solvolysis of O-Nitrochlorobenzene with methanol in the presence of sodium hydroxide.gives O-Nitroanisole. The-reaction may be represented by the following equation:
Reduction of nitroanisole by the iron-acid method gives the corresponding anisidine.
20 kg molten O.N.C.B. and 18 litre methanol are charged in the solvolysis reactor. Temperatrrre is increased to 70-72ºC (reflux-temp). The caustic-methanol slurry which is prepared separately by taking 40 litre of methanol and 5.6 kg of caustic is added slowly in 6 hours duration. After completing the addition, the system is closed and temperature increased to 110ºC. The temperature is maintained for 5 hours. The reaction mass is cooled to 60ºC and it is transferred to distillation vessel, to recover methanol. The residue is treated with 80 litre water, and is stirred for 1 hour. The material is transferred to layer separation vessel. The organic layer is collected in nitro compound feed tank.
Hot water 20 lit is charged in reduction vessel. Iron powder 20 kg is slowly charged with stirring. The temperature is raised to 95-98ºC. 400 gm acetic acid is added. O-nitroanisole is added at the reflux temperature in about 3-4 hours. Add 400 gm acetic during this period. After completing the reaction the reaction mass is cooled to 80ºC.
The reaction mass is neutralised, with caustic and then filtered through filter press. The filtrate is collected in layer separation vessel and bottom organic layer is transferred to distillation kettle. The distillation is performed under vacuum (10-20 mm Hg) to collect O-Anisidine.
Colour — yellowish to red liquid
M.P. — 5.2ºC
Boiling Pt — 225°C
Sp. Gravity — 1.09720/4
NITRO-P-ANISIDINE (Fast Bordeaux G P)
After hydrolysis, purify the product to get Fast Bordeaux G.P. Raw materials :
P- acetanisidide 100% -150 Kg.
HNO3 100% as nitric acid(40°Be) - 70 Kg.
Rock salt - 75 Kg.
Chlorobenzene - 375 Kg.
NaOH 100% as 50% - 50 Kg.
Charge chlorobenzene and rock salt into the nitrator. Add 17 Kg HNO3, in 3 hours. Add p-acetanisidide and simultaneously remaining HNO3 at 20 - 30°C, by circulating cold water in the jacket. Stir for 1 hour at 30ºC. Add 16 Kg soda ash for weakly alkaline reaction.
Suck the content of nitrator into the still and distill the chlorobenzene with live steam through the condensor. Separate the condensate in the overflow separator and pass the chlorobenzene to the storage tank.
Cool the mixture to 70°C after removing chlorobenzene. Add sodium hydroxide as 50% solution and dilute with water (upto 1000 litres). Heat the mixture to 70 - 75°C and maintain for 2 hours. Maintain the content alkaline. The hydrolysis is finished when the melting point of a sample is 123 - 124°C. Cool to 30°C blow the content to the vacuum filter, and wash the residue free from alkali.
98 Kg nitro-p-anisidine crude moist.
10 Kg decolourising carbon.
1 Kg soda ash.
Charge 6000 litres water,.decolourising carbon and soda ash in dissolving vessel.
Add nitro-p-anisidine slowly in above kettle. Pass steam for half an hour. Under stirring cool to 40°C. Filter, dry the cake at 95°C under vacuum.
Take 118 kg toluene, 16 kg BON Acid and sodium hydroxide into reaction kettle and stir for an hour at 70ºC Add about 11 kg ortho-anisidine at about 70°C and discharge the mass into enamelled steel condensation vessel.
Add 5 kg phosphorous trichloride in about 4 hours, heat to refluxing temperature for 8 hours. Cool the batch to 80ºC. Charge at this temperature about 1.5 kg sodium carbonate to make product discharge easier and cool to 50ºC. Thereafter cool to 30°C in one hour and to 20-50ºC in one more hour.
After the reaction is completed the whole mass is dumped into a distillation unit. In the distillation, unit steam is passed and 80-90% of the solvent is recovered azeotropically. The residue is tested for alkalinity. Then the slurry is sent to the filter press and the filtrate is discarded.
The cake is washed with hot water, demineralised water and also with water dissolved with dispersing agents. The cake is dried at 80-90°C in drier. After drying, the product is powdered in micro-pulveriser and mixed with urea and packed in steel drums.
Physical Properties of Naphthol AS-OL
Physical Aspect — Dirth white colour
Purity — 95%
Moisture — Below 1%
Naphthol AS G
Naphthol As G is a diacetoacetic toluidide which can be padded on the fiber and developed in the usual manner.
Sulphuric acid monohydrate
Sodium thiocyanate. (anhydrous).
Charge 800 litres of ethylacetoacetate and 205 Kg
O-toluidine in an enamelled vessel. Then add 75 Kg 86% sulphuric acid to form the sulphate. Cool the suspension to 35°C. Add 105 Kg sodium thiocyanate anhydrous and heat the mixture to 90°C for 4 hours and cool. Add 195 Kg sulphuryl chloride and on mixing as the reaction proceeds the temperature rises to 75°C. Maintain the reaction mass at 75-80°C and cool to 30°C and run into a tank containing 1000 litres of water and 750 litres of caustic liquor. Distill the ethylalcohol formed during the reaction. Filter the product wash and dry. Purify the crude product by dissolving in 400 litres water and 450 litres hydrochloric acid at 50°C and filter. The filtrate is made alkaline with caustic liquor. Filter the precipitate, wash and dry. The product is pure Naphthol As G.
METHODS OF MANUFACTURE
The triarylmethane dyes may be produced by several methods, but in general synthesis involves the reaction of two or more species to form a colourless lenco base which is subsequently converted to the colourless carbinol base and finally to the dye. Four general methods of preparation are outlined below for information purposes and to provide the analyst with an insight as to possible impurities which may be present in dye samples. This knowledge will better enable him to select the proper methods of analysis to suit his particular purposes.
One mole of an aromatic aldehyde is reacted with two moles of an aromatic amine to form the leuco base. The leuco base is in turn oxidized to produce the carbinol base which forms the dye upon reaction with acid. This is shown schematically in the following reaction sequence:
This is one of the most important industrial routes since it involves the reaction of a disubstituted diarylketone with an aromatic amine to directly produce the dye without going through the leuco base and carbinol. The scheme is shown in the following reaction sequence illustrating the synthesis of Ethyl Violet:
Aromatic nuclei, even deactivated varieties, are condensed with substituted benzhydrols to form the leuco base which is subsequently converted to the dye. A typical example is shown in the following reaction sequence illustrating the synthesis of Wool Green S:
Diphenylmethane Base Method
Disubstituted diphenylmethane bases are oxidatively condensed with substituted aromatic nuclei to directly form the dye. This is shown schematically in the following reaction sequence:
METHODS OF ANALYSIS
The analysis of triphenylmethane dyes, like that of any other class of dyes, has many facets, the most important of which is the identification of structure type, or class. It is also important to know if the dye is present alone or in mixtures with another dye or dyes. Once these aspects have been determined it is generally not difficult to determine the dye concentration or the concentration of diluents and specific impurities, such as metals, water, acid content, etc. In all of this work it is invaluable to have a series of reference standards whose chemical structures and compositions are well defined. These standards can then be used for many comparisons such as shade, physical properties, and various spectroscopic examinations. The implementation of these analyses requires methods for identification and classification, techniques for separation, and analytical procedures for assay of the dyes and determination of impurities present in the dyes.
Perhaps the first step in the analysis of the dyestuff is to determine whether one dye or a mixture of’ two or more dyes is present. This will aid in the interpretation of further results and guide the choice of techniques. Chromatographic methods could be used to determine if one dye or more dyes are present and these techniques will be discussed below under Separations. There are also several simple qualitative tests which have been used for many years to determine if one or more dyes are present. These are outlined below.
DETERMINATION OF PRESENCE OF MIXTURES OR SINGLE SPECIES
This method is applicable only to dry powdered samples. Wet samples may, of course, be dried prior to employing the technique. A small quantity of dry powdered sample is blown from the tip of a spatula, from a sheet of blotting paper, or any convenient implement onto a sheet of blotting paper or filter paper which has been moistened with water, methanol, or some other appropriate solvent. Alternatively, a flat dish of concentrated sulfuric acid may be used in place of the wet paper. The resulting spots or capillary run outs are visually examined. When a single dye is present the spots will all be the same colour and the run outs will be uniformly coloured, but when more than one dye is present the spots will be coloured differently and the run out will not be uniform. This test can fail if the dye sample consists of perfectly uniform and homogeneous particles.
A small quantity of the dye is dissolved in water or any other appropriate solvent and placed in a small beaker or dish. A narrow strip of filter paper, blotting paper, or even a thread of scoured cotton is suspended over the container so that the lower end is immersed just below the surface of the solution. The dye solution is allowed to rise up paper or thread by capillary action. If a single dye is present, the coloured portion that has risen from the solution will be uniform. If more than one dye is present, the individual components will generally rise at different rates, creating different colour or shade zones.
A small sample of dye is dissolved in water and heated to boiling. A small piece of woolen or cotton fabric is immersed in the solution for several minutes until it is completely dyed. It is then removed from the bath and the dye solution is squeezed back into the bath. This process is repeated with additional portions of fabric until all of the colour is exhausted from the bath. When a single dye is present, all of the dyed fabrics will have the same hue although the depth of shade (intensity or “strength”) will decrease as the dye is depleted from the bath. When mixed dyes are present, the dyed fabrics can generally be divided into two or more lots owing to the different affinities of the dyes for the fabric.
All of the three tests described above can be valuable and are quickly and easily performed. However, they all suffer from the same limitation, namely that the tests will not discriminate mixtures of dyes which have the same colour.
CHEMICAL CLASSIFICATION OF DYES
At the outset of the identification of the dye it should be classified chemically to be sure it is a triphenylmethane derivative. Green (3) developed a procedure which first classifies the dyes into four main groups based on solubility and dyeing properties. The four classes are:
I. Basic dyes and basic mordants: these are water soluble, are precipitated by tannin, and will not dye unmordanted cotton.
II. Direct cotton dyes: these are water soluble, are not precipitated by tannin, and will dye unmordanted cotton.
III. Acid dyes and acid mordents: these are water soluble, are not precipitated by tannin , and will not dye unmordanted cotton.
IV. Water insoluble dyes.
The triphenylmethane dyes fall into both the basic and acid classes, i.e. I and III. They are distinguished from the other dyes in these categories by their behaviour toward reduction and subsequent reoxidation. The principles of this reduction-reoxidation classification are outlined in Table 2. The triphenylmethane dyes are reduced to colourless solutions by the action of zinc and acetic acid. The original colours are not restored by exposure to air but are restored by the action of acid permanganate.
Add 1 g of dye to 100 ml of water and heat to boiling. Soluble dyes will dissolve completely or almost completely.
Take 5 ml of a 0.5-1% aqueous solution of dye. Add 1 ml of a tannin solution prepared by dissolving 10 g of tannin and 10 g of sodium acetate in 200 ml of water. If a precipitate is formed, a basic dye is indicated. If no precipitate is formed, apply the cotton dyeing test.
COTTON DYEING TEST
Boil a piece of mercerized cotton, about 1 in. sq, in 10 ml of a 0.5% aqueous solution of dye for about 1 min. Remove the cotton and boil in dilute ammonium hydroxide for 1 min. If the colour is removed from the cotton, the dye is acidic and is in Class III. If tile colour persists, the dye is a direct cotton dye and is in Class II.
REDUCTION AND REOXIDATION
Add a small amount of zinc dust to 10 ml of 1% aqueous solution of dye. Stir and add a few drops of 5% acetic acid. A complete Change in shade indicates subclass A. No decolourization or a slow and partial one indicates subclass B.If the solution is decolourized, pour some on a piece of filter paper and expose the paper to air. If the original shade of colour returns in a minute or two, the dye is in subclass C. If the colour does not return upon exposure to air, dip it glass rod in a solution of acid permanganate prepared by dissolving 1 g of potassium permanganate and 2 g of concentrated sulfuric avid in 1000 ml of water, and touch it to the paper. Warm gently over a flame to facilitate oxidation. Hold the paper over a bottle of strong ammonia for it few seconds. If the original colour is restored, the dye is in subclass D. If the original colour is not restored, the dye is in subclass E.
CLASSIFICATION OF DYE BY SPECTROSCOPIC TECHNIQUES
The triarylmethane dyes, as a class, absorb very strongly inthe visible region of the spectrum. They are, in general, very intense and brilliant shades and have molar absorptivities about four to eight times that of anthraquinone dyes and about twice that of azo dyes. Generally, the triarylmethane dyes with the highest molar absorptivities are the most symmetrical and have the most basic substituent groups. In almost all cases two absorption bands are prominent in the visible spectra of these dyes. A typical example is shown in Figure 1. The greater intensity band, the X band occurs at longer wavelengths than the smaller Y band. In some cases, as with the Crystal Violet family, the X and Y bands overlap to form a single peak with a shoulder on the short wavelength side; this is illustrated in Figure 2. For this reason intensity data are sometimes presented by only reporting the value of the X band although in many cases both bands are reported. Absorption values of many triarylmethane dyes along with other physical properties can be found in Reference 5. A select few are listed in Table 3.
Table 3. Visible Absorption Bands for Triarylmethane Dyes in Water
Compound X band Y band
Malachite Green 621 427.5
Fuchsine NJ 543.9 487.1
Magenta P (CI 42510) 546.5 489.2
Methyl Violet 587.0 535.0
Crystal Violet 591.0 540.5
Ethyl Violet 596.9 546.5
Red Violet, 5RS (CI 42690) 551.3 419.3
Eriochrome Cyanine R (CI 43820) 587.5 544.5
Victoria Blue B 619.2 567.0
Wool Green S 634.1
Erioglavcine A 639.0
Brilliant Green 623.0
Setocyanine (CI 42140) 612.3
Setoglaucine 630. 8
The characteristic visible absorption bands and the high molar absorptivities can many times be sufficient to qualitatively identify the dye as belonging to the triarylmethyl class. A good collection of standard (pure) dyes is extremely helpful at this point because this is perhaps the simplest and fastest way to identify the dye. The absorption spectrum can now be compared to spectra of the standards. Although matching spectra does not guarantee that the dyes are identical, it is usually safe to conclude that, if two dyes have identical spectra in several solvents (three or more) the dyes are either identical or very similar.
These rules may be helpful in assigning positions or substituent groups.
Ultraviolet and infrared spectroscopy can also be used either directly on the dye or on the dye precursors, i.e., the carbinol base or the leuco base. UV analysis is not too useful except for the comparison of the minima and maxima observed to that of known standards. IR spectroscopy can, of course, also be used in this comparative manner. However, further information can be obtained as to the presence or absence of functional groups and in some cases the position of substitution on the aromatic rings, ie, ortho, meta, or para, can be determined. IR is probably most valuable in the elucidation of the structure of triphenylmethane dyes when considered with elemental analysis. The data from the combined techniques can be used to rule out the presence of functional groups.
Nuclear magnetic resonance and mass spectrometry can sometimes be useful aids in elucidating the dye compound, especially when used in conjunction with IR and elemental analysis. These techniques are best performed on the leuco bases of the triarylmethane dyes. In favourable cases, the combination of all those techniques can determine the nature of the aryl groups, i.e., phenyl, naphthyl, etc, the nature of the ring substituents, and the position or positions of substitution.
ELECTROCHEMICAL CLASSIFICATION OF DYES
Many electrochemical studies have been performed on the triarylmethane dyes but no test has yet been devised as a class identification technique. However, voltammetric techniques can be valuable in identifying the class of dye and the individual dye.
Galus and Adams, (11) studied the anodic oxidation of alkylmaniosubstituted triphenylmethane dyes. These dyes, eg; Crystal Violet (VI), had characteristic cyclic viltammograms can be seen in Figure 3 where the first anodic and cathodic sweeps are shown by the lighter line. As can be seen, a reduction wave, C´, is obtained on the cathodic sweep. On all subsequent sweeps, the reversible CC´ at about +0.55 V persists owing to the formation of a stable product which resulted from the first anodic oxidation. Features of this type can be used in conjunction with absorption spectro-scopic methods mentioned earlier arid with known standards to more positively identify individual compounds.
Berg presented data on a comparative polarographic study of dyes which indicated that the different dye classes reduced in rather narrow potential regions. The oxazines, thiazines, and phenazines reduced from -0.15 to -0.6 V, the azos from -0.45 to -0.65 V, the anthraquinones from -0.45 to -0.75 V, the triphenylmethanes from -0.64 to -0.70 V, and the xanthenes from -0.73 to -0.86 V. This indicated that the technique might be applicable but, later data by Nemcova and Memec showed that the, triphenylmethanes, reduce at from -0.19 to -0.8 V, depending on the substituent groups. Thus, the technique is for from absolute but can be useful in comparative tests.
CLASSIFICATION OF DYES ON FABRICS
The methodology used in this scheme (3-4) is similar in principle to that, described above for the chemical classification of dyes in substance, ie, the dye itself and not an applied sample. The classification is made by first boiling the dyed fabric for 1-2 min in 1% acetic acid to remove basic dyes. The extracted dye is then tested with tannin and cotton as described above. The dyed fabric is next boiled in 1% ammonium hydroxide for 1-2 min. This treatment removes acid and direct dyes. The extract, is divided into two portions. One portion is acidified and boiled with some wool and mercerized cotton; the other portion is boiled to remove the, ammonia after which a little salt, wool, and mercerized cotton are added. The results from those tests allow the classification to be made as described above.
The Subclassification is similar to that described above but utilizes different reagents for reduction and oxidation; the reagents and procedures are described below.
Formosul G. Dissolve 20 g of Formosul G (sodium formaldehyde sulfoxylate) in 75 ml of hot water and dilute with 75 ml of cold water and 50 g of mono or diethylene glycol.
Dissolve I g of ammonium persulfate and 0.5 g of ammonium dihydrogen phosphate in 100 ml of cold water.
DETERMINATION OF SUBCLASS
Boil the sample, a piece of dyed material about ½ in. sq, for about 0.5-1 min in a test tube with Formosul G reagent. Rinse the sample, if decolourized, thoroughly with tap water and allow to dry on white paper for about an hour. If the colour is restored, the dye belongs to subclass C. If the colour is not restored, heat the sample to boiling in a test tube containing a little water. Add the developer dropwise and avoid an excess. If the colour is restored, the dye belongs to subclass D, ie, contains triphenyhmethane dye. If the colour does not return, the dye is in subclass E.
Once the dye is stripped from the fiber by the above procedure or by appropriate solvent, the identification need not be made by the above chemical system. The isolated dye can also be examined by the other procedures described previously in order to identify its class and perhaps the dye compound itself.
Separation methods span all of the methods and techniques described in this chapter. Paper, column, and thin-layer chromatography can be extremely valuable in the analysis of all types of dyes including triphenylmethane and related dyes. Gas chromatography will not be considered because of the lack volatility of the triphenylmethane make them impossible to analyze in this manner although it might be possible to analyze their leuco bases by this technique. The chromatographic liquid-solid adsorption methods can be used to separate a multitude of dyes of all classes. Thus they may be used in lieu of the simple techniques to determine if a sample consists of a single dye or dye mixture. The Association of Public Analysts published an identification schema for dyes permitted in foods based on the work of Tilden, the National ChemicalLaboratory, and Rutter using paper chromatography. This work indicates that, separation techniques can be used in lieu of the chemical classification system and when coupled with visible or infrared absorption spectroscopy, or electrochemical behaviour will allow rapid identification.
Once the dye in question has been separated from other species present, it can be eluted from the chromatographic support for spectrophotometric assay which is free of interferences, or can be measured directly on the support, in the case of paper or thin-layer chromatography via spectrodensitometry as described by Ganshirt and many other. In the cases just, described the separation techniques are used for assay purposes.
Finally the samples can be separated on a large or preparative scale and eluted for the purpose of isolation of pure dyes as primary reference standards. In all of the separations described below, it should be remembered that the techniques may be used for all aspects of the dye analysis, ie, determination of presence of mixtures, identification, assay, determination of impurities, or preparation of pure standards.
The identification scheme published by the Association of Public Analysis for food dyes utilized Whatman paper and six different solvent systems. Their method involves chromatographing an unknown dye along with suspected known in the six different chromatographic systems. If the unknown has the same R1 value as a known in all six of the systems it can be inferred, rather safely, that the two dyes are identical. The solvent systems employed are: 1.1:99 (v/v) mixture of ammonium hydroxide (sp gr 0.880) and water; 2.2.5% (w/v) aqueous sodium chloride solution; 3.2% (w/v) solution of sodium chloride in 1:1 ethanol; 4. 1:1:2 (v/v)water-isobutanol-ethanol mixture; 5. 5:12:20 (v/v) glacial acetic acid-water-n-butanol mixture; 6.99:1 mixture of 2:2:3 (v/v) water-ethanol-isobutanol and ammonium hydroxide (sp gr 0.880).
Table 5 lists the Rf values for a series of dyeschromatographed in these six systems. As can be seen fromthesedata the wide variations in Rf values are helpful for identification and for analysis.
Paper chromatography was also employed for separation of triphenylmethane dyes by Dobas and Gasparic and Matrka. Some of the data of the latter authors is presented in Table 6 as an indication of separations that can be obtained by paper chromatography. In this case a liquid-liquid system was employed. The paper was impregnated with various amounts of lauryl alcohol and four different solvent systems were used. The systems employed were: 7. Paper impregnated with 2% lauryl alcohol. Solvent: 2:2:1 (v/v) ethanol-ammonium hydroxide-water; 8. Paper impregnated with 5% lauryl alcohol. Solvent: 2:2:1 (v/v) ethanol-ammonium hydroxide-water, 9. Paper impregnated with 5% lauryl alcohol. Solvent: 1:1 (v/v) ethanol-ammonium hydroxide. 10. Paper impregnated with 5% lauryl alcohol. Solvent: 1:1 mixture of 25% ethanol and a 5% aqueous potassium chloride solution.
Although thin-layer chromatography was developed as early as 1938, it did not become popular till the late 1950s-early 1960s. Stahl’s book is a detailed reference for the technique itself and for the application to dyes. The publications of Rettie and Haynes and of Saenz Lascano Ruiz and LaRoche also deal with the application of this technique to dyes. Thin-layer chromatography is a powerful tool because it has excellent separation or resolution capabilities, it is rapid, lends itself well to preparative work, and is rather easily quantitated via extraction and subsequent analysis or directly by scanning spcctrodensitometry. It can be used for identification and classification, quantitative determinations such as impurities in the dye, assays of the dyes, and control tests for reaction completion.
The basic triarylmethane dyes have been studied by several authors. Malachite Green and Methyl Violet were separated on silica gel G using a mixed solvent of 9:1:1-butanol-ethanol-water Fuchsine, Rhodamine B, and Rhodamine 6G (CI 41560) can be separated from each other using silica gel G and a mixed solvent of 4:1:5 ç-butanol-acetic acid-water. Naff and Naff separated Victoria Blue, Methylene Blue (CI 52015), Crystal Violet, Rhodamine B, and Malachite Green on microscope slides coated with silica gel using 2:2:1 methyl ethyl ketone-acetic acid-isopropanol as the solvent. The respective Rf values were 0.30, 0.02, 0.20, 0.43, and 0.12. Takeshito et at. separated many food dyes on polyamide layers using 4:1 carbon tetrachloride-methanol. Among these were the triarhylethane dyes, Fuchsine Base, Crystal Violet, Ethyl Violet, Malachite Green, and Night, Blue (CI 44085). These respective Rf values were 0.23, 0.54, 0.56, 0.91, and 0.49. Stier and Specht, separated basic dyes of the xanthene class on silica gel using 4:1 -propanol-formic acid, and other mixed solvents. The dyes included Rhodamines B, G, and S, and Pyronine G (CI 45005). Contaminants in some of these basic dyes were studied by Logar et al. on silica gel G using 2:1:5 -butanolacetic acid-water as the solvent.
Many of the acid triarylmethane dyes have also been studied. The separation of many of these dyes in ink was reported on silica gel G using 4:1:5-butanol-acetic acid-water as the solvent. Druding also separated dyes in ink using silica gel G and 95% ethanol as the solvent while Rettic and Haynes used 60:20:20:0.5 h-butanol-ethanol-water-acetic acid and Perkavee and Perpar used 2:1:5 -butanol-acetic acid-water.
The sulfonphthaleins, which are used as acid-base indicators, were reported to be separated on silica gel G using the following mixed solvents: 50:45:5 amyl alcohol-ethanol-concentrated aqueous ammonia, 6:3:1 ethyl acetate-pyridine-water, and 60:40:1 benzene-isopropanol-acetic acid.
Anwar et al. used electrophoresis with a cellulose acetate membrane as the supporting medium, and a 1:1 mixture of 0.1 M sodium acetate and isopropyl alcohol adjusted to pH 4.6 with acetic acid as the buffer to separate a group of food dyes. Among these were the triarylmethane dyes FD&C Blue No. 1(CI 42090), Green No. 1 (CI 42085), 2 (CI 42095), and 3 (CI 42053), Violet No. 1 (CI 42640), and Red No. 3 (CI 45430).
LIQUID COLUMN CHROMATOGRAPHY
Several authors have presented work on the column chromatography of food colours including triphenylmethane and related dyes. The work of McKeown and Thompson serves as illustration of the types of separation one might achieve by liquid column chromatography. In this paper the authors studied the behaviour of Fast Green FCF (CI 42053), Brilliant Blue FCF (CI 42090), Light Green SF Yellowish (CI 42095), Guinea Green B, and Benzyl Violet 4B. A 100 mm X 15 mm alumina column packed by aqueous slurry was used. Samples were applied in 0.1 N acetic acid. The column was then washed free of acid by water and developed with various concentrations of pyridine in water. The authors found that 5% pyridine in water provided a rapid transport through the column but gave poor resolution of the dyes studied. A 0.5% pyridine in water carrier gave excellent resolution but took impractical lengths of time to completely elute the dyes. Compromise mixtures were used to perform their separations but a gradient elution technique would be appropriate. As an example, the position of the dyes in the 100-mm long column, using a 1.5% pyridine in water carrier, are:
Dye Height in column, mm
Fast Green FCF 20-28
Light Green SF Yellowish 22-34
Benzyl Violet 4B 36-50
Guinea Green B 55-75
Brilliant Blue FCF 56-72
Liquid column chromatography has the advantage of being able to use larger sample sizes thus providing enough sample for analysis and for preparative use in a shorter period of time.
The quantity of dye in any given sample is generally determined by direct measurement of concentration or by comparison with a standard. The type of method used depends on the information desired and the extent, to which correlation exists between the analytical method and the final use of the dye.
Titration With Another Dye
In dilute aqueous solution, strongly acid and strongly basic dyes will generally react to form a precipitate. This was used as the basis of a quantitative test by Brown and Jordan. These authors found that: 1. The two dyes must have distinctly different colours, eg, red and green, blue and yellow; 2. The acid dye should be added to the basic dye and reverse addition seldom yields good results; 3. The decision on the determination of the end point must be made quickly. In performing this analysis, the solution concentrations are generally about 1 g/liter. The end point is detected by spotting a piece of filter paper and is not always definite. In these cases the dye to be determined is titrated with a solution of dye and tannic acid in the presence of sodium acetate. This gives a more granular precipitate which settles more quickly. As an example, the determination of Malachite Green is given.
Prepare a 0.2% solution of Malachite Green and titrate a 25-ml aliquot with a solution containing 1g of Orange II, 2 g of tannic acid, and 2.5 g of sodium acetate per liter until no further precipitate is formed upon the addition of the titrant. The exact end point is indicated by the appearance of an orange ring on filter paper.
Some other dyes which can be determined in this way, the titrants, and the characteristic end points are listed in Table 7.
The reactions between two dyes are often not stoichiometric and therefore the titrant has to be standardized with a pure dye sample.
Titration as an Acid
Many of the triphenylmethane dyes can be assayed by titration as an acid. This is generally accomplished by the addition of a known amount of excess alkali which precipitates the carbinol base. The carbinol base is removed and the excess alkali is backtitrated with standard acid. The success of this method depends on the nature of the particular dye and its ring substituent’s, eg; some substituent’s may also titrate or reduce the effect of the acidity of the dye molecule. As an example, the determination of Crystal Violet is given below:
DETERMINATION OF IMPURITIES
In many cases it is important to determine the impurities in the dye samples.The spectroscopic, chromatographic, and electrochemical techniques described earlier are best employed for determining impurities which themselves are dyes or dye precursor. The determination of species which are unrelated to the dyes are mentioned here.
DETERMINATION OF METALS
The methods available for the determination of metals in dyes are much too numerous to list. A good source for methods of this type is the work of Clayton. Examples of the polarographic and titrimetric determination of zinc in Crystal Violet are given below.
POLAROGRAPHIC DETERMINATION OF ZINC IN CRYSTAL VIOLET
Dissolve 0.5g of sample of the dye in water and dilute to 100 ml in a volumetric flask. Transfer it 10-ml aliquot to a 50-ml flask, add 5 ml of tartrate solution prepared by dissolving 228 go of Na2C4H4O6. 2H2O in 1 liter of water and dilute to volume.
Transfer a 10-ml aliquot of this sample to a 50-ml volumetric flask, add 5 ml of a 20% sodium hydroxide solution, dilute to 20-25 ml, and heat for 5 min on a steam bath. Cool to room temperature, add 0.5 ml of gelatin solution prepared freshdaily by dissolving 0.25 g of gelatin in 25 ml of warm water, and dilute to 50 ml. Start the polarogram at 1.20 V vs S.C.E and scan to atleast -1.85 V vs S.C.E. Determine the diffusion current of the zincwave, half-wave potential -1.57 V vs S.C.E., by normal techniques. Run a reagent blank in the same manner. Calibrate by preparing standard, which contain 50 mg of dye plus 2.0, 1.5, and 1.0 mg, of zinc. Determine tile diffusion current per mg of zinc which should agree within ± 1% for this series of standards.
where Is = diffusion current for the sample, in µA
IB = diffusion current for the blank determination, in µA
F = diffusion current per mg of zinc as determined from standards
W = sample weight, in mg (=50)
TITRIMETRIC DETERMINATION OF ZINC IN CRYSTAL VIOLET
Prepare a potassium ferrocyanide solution by dissolving 42.3 g of K4Fe(CN)62H2O in a 1 liter volumetric flask in water and diluting to volume. Prepare a zinc sulfate solution by dissolving 15.35 g of ZnSO4·7H2O in water in a 500 ml volumetric flask and diluting to volume. Standardize the former solution by pipeting 50 ml of the zinc sulfate solution into a 600 ml beaker containing 200 ml of water, heating to 80°C, adding 10 ml of concentrated hydrochloric acid and titrating with the potassium ferrocyanide solution as described below. Calculate the amount of zinc in g, equivalent to 1 ml of the potassium ferrocyanide solution; the value should be 0.01 g/ml.
Weigh 1 g of sample into a 50-ml evaporating dish. Add 25 ml of water and place on a hot plate. Heat to first visual bubbling and add, with a pipet, 6 ml of hot 30% sodium hydroxide solution while stirring. Evaporate on the hot plate to a volume of 5-7 ml, cool and filter through a Büchner funnel fitted with a No. 1 Whatman 7 cm filter paper. Wash with 150 ml of water and transfer the combined filtrate and washing to a 400 ml beaker. Rinse the flask with 100 ml of water and add to the filtrate. Heat to 80°C and add 10 ml of concentrated hydrochloric acid. Titrate with the standard potassium ferrocyanide solution, adding it in 0.1 ml portions. Test by adding a drop of the solution to 2 drops of uranyl acetate solution used as an outside indicator on a spot plate. Prepare the uranyl acetate solution by dissolving 25 g of UO2 (C2H3O2)2 2H2O in 250 ml of water. The end point is the first light brown colouration. Allow 2 min before final judgment of end point since some reactions are slow. Carry out a blank determination.
where A = volume of potassium ferrocyanide solution required for sample titration, in ml
B = volume of potassium ferrocyanide solution required for blank titration, in ml
C = sample weight, in g
F = zinc equivalent to the potassium ferrocyanide solution, in g/ml
The above procedure, and those referenced for determining metals in dyes are rather old and outdated. Although there is a scarcity of published methodology for determining metals in dyes by modern techniques there is no doubt that the newer procedures are applicable and are to be preferred. It is suggested that the analyst faced with determining metals in dyes investigate the use of atomic absorption spectroscopy, x-ray analysis, or activation analysis: These techniques are preferred due to their simplicity, sensitivity, and accuracy. The more classical procedures are only recommended when the modern instrumental techniques are not available
DETERMINATION OF ACIDITY
The determination of total free acidity is generally performed by titrating with a standard base to a definite pH in the vicinity of pH7. The result, is generally made as acetic acid. An example of a direct potentiometric titration of acidity in Victoria Blue BO solution is given below. The procedure is also applicable to the determination of acidity in Rhodamine B.
DETERMINATION OF ACIDITY IN VICTORIA BLUE BO SOLUTION. Transfer a portion of the sample to a glass vial fitted with a medicine dropper. Weigh, by difference, 1-1.5 g of sample into a 400 ml beaker. Add 300 ml of water place an air driven stirrer into the beaker and potentionmetrically titrate the sample with 0.3 N sodium hydroxide solution. The end point is indicated by a pH change of 0.4-0.8 at a pH of 7. with 2 drops of titrant.
where A = volume of sodium hydroxide solution used for titration, in ml
N = normality of the sodium hydroxide solution
w = Height of sample used, in g
DETERMINATION OF WATER
Determination of water in powdered samples of dyes can generally be accomplished by Karl Fischor titration. An electrometric end point is generally used due to the coloured nature of the solutions. An example is given below.
DETERMINATION OF TOTAL MOISTURE IN CRYSTAL VIOLET. Standardize the Karl Fischer reagent and the equipment by normal procedures using all automatic titrator and a Karl Fischer type automatic burete. Set the equipment as described in the respective instrument manuals. To the titration apparatus add enough anhydrous methanol to cover the electrodes and titrate the methanol. Add all accurately weighed 1-g sample of Crystal Violet to the flask and be sure that all of the sample drops into the solvent. Stir by magnetic stirrer for 1 min. Titrate and record the volume of Karl Fischer reagent when the “stand-by” light comes on.
where X = volume of Karl Fischer reagent required, in ml
Y = Karl Fischer reagent equivalent found in the standardization
w = sample weight, in g
The salt used for standardizing the dye can usually be determined by salting out the dye with potassium nitrate and testing filtrate for anion. Chloride salts are used in most cases.
Fluorination of any dyes or intermediates can be carried out in side chain or in nucleus. Side chain will be costly, while nucleus fluorination will be cheaper. There are few dyes known with side chain fluorination in colour index, which we will discuss later. Nucleus fluorination is uncovered field, and some research can be done, to develop this branch. Fluorination will increase brilliancy as well as light fastness:
Fully fluorinated compounds have two characteristics :
(i) Low boiling points for compounds of high M.Wt and high densities.
(ii) In addition, they posses excellent electrical characteristics.
Fluorination of any dyes or intermediates can be carried out broadly by two ways.
(ii) Hydrofluoric acid or by any other fluorinated compounds.
Before going into detail of fluorination, first we will study the, merits and demerits of the reagents.
(i) It is a colourless strongly fuming liquid, B.P 19.5°C with an extremely pungent odour.
(ii) Its vapours are corrosive and highly poisonous and often proved fatal. The acid attacks the skin violently and develops painful sores and blisters on being dropped on skin.
(iii) It is highly soluble in water and the aqueous solution above 50% strong fumes strongly in air.
(iv) If forms a constant boiling mixture which boils at 120°C and contains 37% H.F.
(v) It is extremely stable compound.
(vi) Organic compounds are attacked very strongly and destroyed by it.
Materials of Construction
1. Anhydrous hydrofluoric acid attacks neither glass nor any metal except potassium which explodes in contact with it
2. In the presence of even traces of water or moisture, it reacts violently with glass forming silicon tetra fluoride SiF4 and Sodium silico fluoride Na2SiF6, while it dissolves most of the metals with evolution of hydrogen.
3. Lead, silver, gold and platinum are not attacked by the acid, where as copper and nickel get a protective coating of the corresponding fluorides. Lead lines, copper, nickel or teflone i.e. saturated plastics can be used as material of construction in this type of reaction, but the life of the kettle will be short
Actually it is better to use potassium fluoride KF than the hydrofluoric acid.
(1) Fluorine is a pale greenish yellow gas.
(2) It is 1.3 times heavier than air.
(3) It has an irritating and pungent smell and attacks the mucous membrane. It is highly poisonous in nature.
(4) It does not burn in air and oxygen but supports combustions of many elements.
Material of Constructions
It attacks all the metals and forms metallic fluorides. Magnesium, zinc, aluminum, tin, iron, etc. burn on being gently warmed in the gas.
Copper, mercury, lead, and nickel are attacked slowly by fluorine but become coated with protective layer of their fluorides Gold and platinum are not attacked at ordinary temperatures but form their fluorides on heating.
It attacks glass and quartz. The presence of small quantity of moisture acts as a catalyst in these reactions as dry fluorine has very little action on glass below 100°C, provided the glass is kept dry.
Teflon, i.e., saturated plastic is most suitable as material of constructions.
With this back-ground, we will discuss the commercial fluorinated Vat Dyes.
This dye with C.I. No. 61735 was discovered by IG. Fluorination will increase in this dye, brilliancy as well as light fastness.
Due to fluorine, brilliancy as well as light fastness has improved.
Indanthrene Brilliant Violet F3 RK (C.I. 63350)
Its chief merit was “tone in tone” cotton viscose dyeing together with improved wash fastness.
Note:- It is important that the chlorine content 0.1 the fluorine must not be above 1 %, otherwise the dye is dull. The fluorine content of the above dye shall be close to 18.5% and its chlorine content not above 0.5%. If the fluorine content was lower than the above figure, the dye was dull.
This dye is obtained by condensing 1:5 Diamino-4:8 dihydroxy anthraquinone with m-trifluro methyl bensoyl fluoride.
Many types of Vat dyes were made which contained one or more (OCH2CF3) group in aryl nucleii. In general these new classes were slightly weaker tinctorially than the corresponding methoxy derivatives with no improvement in fastness properties, shades of trifluoromethyl ether products were hypsochromic versus the methyl ether analogues.
They were dull blues, with good fastness properties. They were not fast to alkaline wash.
A trifluoromethyl derivative of Indanthrene yellow 6GD was found inferior in light fastness to the standard.
Indanthrene Blue CLB
It is used for curtaining, furnishing and heavy shades for awnings. It can also be used for goods, to be bleached for direct and discharge prints and for resist styles. Light fastness is very high about 8.
Indanthrene printing blue HFG
It can be synthesised as follows :
Indanthrene printing blue HFG has a shade slightly greener and brighter than indanthrene blue GCD, but it is distinguished from this and other printing blues of brilliant shade by its excellent light fastness and very good washing fastness. It is not sensitive to sodium dichromate and acetic acid when used for fastening the development of the print. It has superior fastness to perspiration than indanthrene printing blue B, GG or R.
C-H bond distance is 1.09A°, while the C-F is 1.36A° and C-CI is 1.76A°. It is also seen that the C-C bond distance is 1.54A°. It is obvious that fluorine is unique, in than it is the only halogen which has a bond radius less than that of the C-C distance, hence it can replace hydrogen in essentially all hydrocarbon structures without distorting the normal carbon to carbon bonds. In addition, of course, these bonds are extremely stable.
So with proper development of method nuclear substitution is possible and then following dyes can be directly fluorinated which will give higher fastness and brilliancy.
In general it was found that the exchange of a trifluro-ethoxy group for the alkoxy caused the colours to assume a lighter hue. This is particularly noticeable with the dihydroxy dibenzanthrone derivative in which green colour turns blue. The exception is the thioindigo whose colour depends even when changing from orange to scarlet
Fluorinated compound or fluorine can be used for the, production of fluorine containing fast bases, pigments azo dyes, Disperse dyes, for cellulose acetate, Vat dyes, surfactants etc.